Table of contents

Non-inductive full current drive (CD) in a reversed magnetic shear (RS) plasma with high energy confinement of 1.4 times the H-mode scaling at 82% of the Greenwald density has been achieved simultaneously for the first time in a tokamak; these results were obtained in the JT-60U tokamak. The full-CD state was realized by a combination of waves in the lower hybrid range of frequency and neutral beam injection together with high bootstrap current fraction (f_bs) of 62% to the total plasma current. The combination has demonstrated for the first time active current profile modification in a high f_bs RS plasma both in the core and in the peripheral regions.

The electron-retarding range of the current–voltage characteristic of a flat Langmuir probe perpendicular to a strong magnetic field in a fully ionized plasma is analysed allowing for anomalous (Bohm) cross-field transport and temperature changes in the collection process. With probe size and ion thermal gyroradius comparable, and smaller than the electron mean free path, there is an outer quasineutral region with ion viscosity determinant in allowing non-ambipolar parallel and cross flow. A potential overshoot lying either at the base or inside the quasineutral region both makes ions follow Boltzmann's law at negative bias and extends the electron-retarding range to probe bias eϕ_P∼+2T_∞. Electron heating and cooling occur roughly at positive and negative bias, with a T_e-minimum around eϕ_P∼−2T_∞; far from the probe heat conduction cools and heats electrons at and radially away from the probe axis, respectively. The potential overshoot with no thermal effects would reduce the electron current I_e, making the ln I_e versus ϕ_P graph downwards-concave, but cooling further reduces I_e substantially, and may tilt the slope upwards past the temperature minimum. The domain of strict validity of our analysis is narrow in case of low ion mass (deuterium), breaking down with the ion Boltzmann law.

The results of analytical solution of time-dependent two-dimensional full-wave equation are presented here for the case of fluctuation reflectometry. We consider a family of plasma density profiles with cylindrical symmetry, which allow for an integration of the microwave scattering equation.

Based on the results of analytical modelling, Doppler reflectometry as a method of plasma velocity measurements is investigated. It is shown that the accuracy of Doppler shift measurements is sensitive to the spectral content of the broadband turbulence, as well as to the curvature of the plasma cut-off layer. Addressing the problem of fluctuation amplitude measurements, it is suggested that a power-related parameter of the microwave signal, called fluctuation index, is suitable to quantify the plasma turbulence level provided that the fluctuation spectrum is a priori estimated. As an illustration, we apply our model to estimate the density fluctuation level in the edge plasma of the JT-60U tokamak.

The transition from linear to non-linear regime of fluctuation reflectometry is followed within a one-dimensional WKB model. The general case of statistically inhomogeneous turbulence and arbitrary density profile is investigated analytically. It is shown that in spite of the fact that the signal frequency broadening in both regimes is determined by poor localized small angle scattering, in non-linear regime radial correlation reflectometry provides highly localized information on the turbulence characteristics.

High β, good confinement and stability properties of the low aspect ratio tokamaks, or spherical tori (ST), have been predicted theoretically and preliminarily confirmed in several large experiments recently. This paper reports on impurity transport experiments carried out in ohmically heated plasmas of the small spherical torus CDX-U with the aspect ratio of A≃1.5. Vacuum ultraviolet and soft x-ray multichannel spectroscopic diagnostics are used to measure intrinsic carbon, oxygen and radiated power radial brightness profiles in plasmas with T_e(0)≃60–80 eV and n_e(0)≃2×10¹³ cm⁻³. The measurements are performed in both magnetohydrodynamically dominated and quiescent phase of the plasmas. The properties of the observed low m/n modes, sawtooth oscillations, and ST-specific reconnection events are discussed in the context of particle transport. The measured impurity profiles are modelled using one-dimensional impurity transport code MIST and a collisional-radiative package CRMLIN. Impurity diffusion of 0.2 m² s⁻¹⩽D⩽0.6 m² s⁻¹ and convection velocity of v≃4–6 m s⁻¹ are inferred from the modelling. These transport coefficients are very close to the neoclassical theory predictions obtained with the FORCEBAL code, which uses analytical plasma viscosity expressions valid for an arbitrary aspect ratio geometry. Neoclassical analysis indicates that both carbon and oxygen are in the collisional regime, and the Pfirsch–Schluter flux is the major fraction of the impurity flux. The causes of the observed strong non-diffusive transport are discussed, and it is concluded that the ∇n_i/n_i term, resulting from highly peaked ion density profile, makes the largest contribution to the inward pinch. Present analysis suggests that drift wave turbulence is reduced in CDX-U ohmically heated discharges within at least r/a⩽0.4, however more refined measurements are needed to interpret the results in the framework of ST ion transport.

The strong flows of relativistic electrons, encountered in high power short pulse laser plasma interaction, have the potential to drive Weibel instability producing short wavelength magnetic fields. The presence of a Langmuir wave couples the Weibel instability to two electrostatic wave sidebands. The density perturbations due to the sidebands couple with the oscillatory electron velocity due to the pump to produce a current that enhances the growth rate of Weibel instability. The thermal spread of the energetic (drifting) electrons, however, reduces the growth rate.

Two significant problems that need to be solved for any future fusion device are heat removal and particle control. A very promising method to attack these problems in tokamaks and helical devices is the use of a divertor, providing a controlled interaction zone between plasma and wall. By carefully designing a divertor, conditions can be created in front of the divertor targets, which lead to a sufficient reduction of the power load on the targets by strong radiation redistribution. Any solution of course needs to allow for an energy confinement which is at least sufficient for the realization of a fusion reactor. Since energy confinement has been found to be strongly related to edge anomalous transport and edge plasma profiles, the ultimate aim is to find an integral solution which is optimum with respect to exhaust, heat load and energy confinement.

Two different types of divertors are presently being investigated in helical devices: the `helical divertor' and the `island divertor'. So far divertor concepts have been investigated only in a few helical devices. Theoretical and experimental efforts have mainly concentrated on the suitability of divertor magnetic field structures, while detailed studies of the divertor plasma properties for the two types of divertor configurations have only recently begun. In the course of this exploration, a promising new high-density H-mode (HDH) plasma operational regime has been discovered on the Wendelstein stellarator W7-AS. It benefits from high-energy (up to twice the value of the International Stellarator Scaling ISS95) and low impurity confinement times, complemented by edge radiated power fractions of up to 90% in detached regimes. This allowed quasi-steady-state operation for up to 50 energy confinement times and so far was only constrained by machine operability.

The integral constitutive relation for high frequency waves propagating in toroidal axisymmetric plasmas, obtained by formal integration of the linearized Vlasov equation, is simplified assuming the range of spatial dispersion to be small compared to the linear dimensions of the plasma. We propose to call this the `quasi-local approximation'. A (formally infinite) system of purely differential wave equations is obtained, which should be a good approximation under conditions similar to those which would justify an Eikonal Ansatz for the form of the wave fields. This system is valid to all orders in the Larmor radius, and, in the presence of a poloidal static magnetic field, predicts a different plasma response to each poloidal Fourier component of the h.f. field. Compared to ray tracing based on the Eikonal approximation, these wave equations have the advantage of allowing to take into account periodicity, boundary conditions, and toroidicity-induced coupling between poloidal Fourier modes.

As an example, the quasi-local wave equations are used to model propagation and absorption of the compressional wave at frequencies higher than the ion cyclotron frequency in the high-β plasma of the National Spherical Tokamak Experiment in Princeton, USA. Because of the low magnetic field and the tight aspect ratio of this device, large Larmor radius effects and toroidicity play an important role in these experiments. This example, therefore, illustrates well the importance of taking into account these effects, and, in particular, the different response of the plasma to each poloidal Fourier mode.

This paper documents critical issues in internal transport barrier (ITB) plasmas in JET, namely the transition from type III to type I edge localized modes (ELMs), and the subsequent impact of large amplitude ELMs on the ITB. Benign type III ELMs are observed in ITB plasmas at input powers much larger (up to a factor 3) than the empirical threshold for type III/I transition derived from standard H-modes. Various measurements indirectly suggest a larger fraction of plasma current at the edge of ITB plasmas. Experimental results look consistent with a type III ELM regime controlled by a large fraction of edge current. Especially, the transition to type I ELMs does not occur for a broad current profile (ℓ_i≈0.78) characterized by low edge magnetic shear (s₉₅≈2.6), and a back transition from type I to type III has been found to well correlate with an increase of the edge current. When large ELMs occur, strong perturbations δT_e on electron temperature are generated, and propagate inwards on a ballistic timescales, at v_burst≈160 m s⁻¹. It is of the order of one third (respectively one ninth) of the curvature (respectively diamagnetic) drift. This propagation looks reminiscent of non-local transport experiments. The perturbation induced by large ELMs can reach the ITB. In this case, δT_e increases in the vicinity of the ITB before being strongly damped further inside. Such ELMs also lead to a transient increase of T_e gradient at the ITB, which then moves inwards on a diffusive timescale (χ≈3×10⁻² m² s⁻¹) while degrading.

The influence of trapped electrons on ion-temperature-gradient driven plasma instabilities is investigated in the bumpy pinch configuration as a simplified model of the Wendelstein 7-X stellarator. For these investigations a gyrokinetic global linear particle-in-cell simulation of the plasma ions and electrons was done with the GYrokinetic Global Linear Equation Solver (GYGLES) code (Fivaz M et al 1998 Comp. Phys. Comm. 111 27, Hatzky R and Fivaz M 1998 Proc. EPS 25th Conf. on Controlled Fusion and Plasma Physics (Prague, 1998) (Petit-Lancy: European Physical Society) p 1804). For this purpose, the code was modified by replacing the adiabatic response of the electrons with a gyrokinetic treatment. The growth rates of the modes considered were found to increase owing to the trapped electrons and the gradient of the electron temperature. However, the effect is small in relation to that of trapped electrons in typical tokamak configurations.

In this paper, the algorithms to determine the safety factor q and density profiles in real time are presented. They have been designed to be implemented in JET, in the framework of the real time enhancements for the control of the internal transport barriers. In this method, only the signals of the magnetic diagnostics and of the interferometer polarimeter are used. Instead of solving the equilibrium equation, which at the moment does not seem fully compatible with real time requirements, the topology of the magnetic surfaces is determined on the basis of the external magnetic measurements. Then the density and poloidal field profiles are found inverting the interferometric and polarimetric measurements, respectively. Once the poloidal field has been obtained, the q profile is inferred. The density profiles obtained have been checked against the JET LIDAR diagnostic, showing very good agreement. On the other hand, the q profiles have been compared with the results of several versions of the equilibrium code EFIT, proving that the algorithms developed can identify with good precision both monotonic and reversed shear profiles. An optimized version of the code has also been written and it has been verified that the computing time is compatible with the requirements of JET real time system. The present algorithm is being implemented and is used during JET experimental campaigns.